Page 233 - Distillation theory

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6.9 Complexes of Heteroazeotropic and Heteroextractive Distillation 207

triangle at some line called the vapor line, in the concentration tetrahedron at the
vapor surface etc. (if x ∈ Reg L1−L2 then y ∈ Reg vap ).
We examine the most typical splits and separation sequences for various types
of three-component mixtures (Fig. 6.16 shows examples of heteroazeotropic dis-
tillation; Fig. 6.17 shows examples of heteroextractive distillation).

6.9.1. Heteroazeotropic Distillation
Figure 6.16a shows separate usage of distillation column and decanter, when top
product of the column, close in composition to ternary heteroazeotrope–unstable
node, is directed after cooling to decanter. The example is separation of the mix-
ture ethanol(2)-water(3) using toluene(1) as entrainer (Pilhofer, 1983). At such se-
quence, the structure and evolution of section trajectory bundles remain the same
as at separation of homogeneous mixtures, when one of the products is azeotrope –
unstable node – and the second product is pure component (see Chapter 5). The
difference from homogeneous mixtures consists of the fact that the point of vapor
composition from column y D should lie in the region of existence of two liquid
phases at the vapor line. The distillation trajectory looks as follows:

x B → S s → x f ⇐⇓ x f −1 → y D ≈ N Haz
Reg Reg t Reg sh,R Reg Reg
B s sep,s att vap
Figure 6.16b shows joint usage of a distillation column and a decanter, when one
of two liquid phases is brought in to the reflux of the column from the decanter or
some amount of the second phase is added to the first phase. The example is sep-
aration of the mixture isopropanol(2)-water(3) using benzene(1) as an entrainer
(Bril et al., 1977). Figure 6.16c shows another variant of distillation column for
this separation, with one bottom section.
In these cases, one of two liquid phases is brought into the more minimum
reflux of the column from the decanter, therefore the necessary number of trays is
finite. In other cases, the reflux with one phase is not sufficient for the separation,
which makes necessary reflux with mixture of both phases.
In contrast to distillation of homogeneous mixtures, it is not expedient for the
heteroazeotropic complex to carry out calculation of minimum reflux mode before
calculation of the necessary number of trays. It is offered in Chapter 7 to carry out
at the beginning calculation of the necessary number of trays at reflux with one
phase and then, only if this reflux is not sufficient, to determine necessary flow
rate of reflux with both phases.
The important peculiarity of the sequence at Fig. 6.16b consists in the fact that
column section between the cross-section of feed input and the top of the column
by its nature is not the top but the intermediate section of the column with two
feeds (see Section 6.3.). This leads to important peculiarities of the trajectory
bundle (Reg R ) of this section.
w,int
As we saw before, the location and direction of trajectories of intermedi-
ate sections differ from those of trajectories of the top and bottom sections. In